1. Field of the Invention
The invention relates to phased array detection systems and radio communications systems.
2. Description of Related Art
Phased arrays are used in radar systems to receive electromagnetic signals that are transmitted from or reflected from objects at a distance. Phased array systems employ a plurality of sensors distributed over a surface which detects the electromagnetic signals from the sources. Array geometries can be either one or two dimensional. Cellular telephone systems may employ phased arrays to receive signals emanated from distant transmitters. Ultrasound imaging systems may employ phase array receiver to locate reflected ultrasound. In a digital phased array system, the signals incident upon the phased array are sampled and digitized at successive instants in time for each of the sensor elements. The digitized signals are processed in order to produce solutions estimating the direction of arrival (DOA) of signals transmitted or reflected from the sources.
The digital signals detected by the sensors can be processed by superresolution analyzers. Superresolution analyzers in the spatial domain solve for the direction of incoming narrow band signals from sources that have small angular separations, usually less than a Rayleigh resolution. The Rayleigh resolution for a narrow band electromagnetic signal of central wavelength .lambda., incident on a linear array of length L, is given by .lambda./L. For a linear array with M elements each separated by .lambda./2 the Rayleigh angular resolution is .delta..theta..apprxeq.2/M.
Significant advances have been made in recent years in the development of superresolution analyzers employing phased arrays that can accurately estimate the DOA of point sources. Electromagnetic or acoustic waves arising from distant sources behave as plane waves incident upon the array. Near optimal performance results for the DOA estimates are exhibited by the subspace array processing algorithms such as, for example, the MUSIC algorithm (See "Multiple Emitter Location and Signal Parameter Estimation", R. O. Schmidt, IEEE Trans. Antennas and Propagation, Vol. AP-34, pp. 276-280, March 1986), the Root-MUSIC algorithm, (See "The Mathematical Basis for Element and Fourier Beam Space MUSIC and Root-MUSIC Algorithms", S. D. Silverstein and M. D. Zoltowski, Digital Signal Processing, Vol. 1 pp. 161-175, July 1991), and the ESPRIT alogorithm (See "ESPRIT--A Subspace Rotational Approach to Signal Parameter Estimation", R. Roy, A. Paulraj, and T. Kailath, IEEE Trans. Acoust. Speech Signal Process vol. ASSP-34, pp. 1340-1342, October 1986).
Although the present superresolution analyzer compute the DOA of sources, they do not compute their relative signal strengths or the signal to noise ratios of the sources. In radar applications this information may be useful in reducing false alarms in the identification of real targets. This information is especially important for discerning sources that emit signals of relative low power that have bearing angles that are in close proximity to the bearing angles of sources that emit signals of relative high power such as electronic countermeasure jammers.
In communication applications, it is important to know both the DOA and relative powers of communicating sources to select the correct source to decode a proper message.
Currently, there is a need for a method that determines both the DOA of sources as well as their relative signal strengths.